Bulk Export: Allow exporting to one directory per tag value instead of one directory per combination of values

Is your feature request related to a problem? Please describe.

I regularly tag libraries with people (e.g., Person|John, Person|Jane).
That way, I can export images to directories for all the people, and easily provide each with pictures of them:

export file name: $(FILE_FOLDER)/darktable_exported/$(CATEGORY[0,Person])/$(FILE.NAME)

However, as soon as multiple people appear in an image, I get a directory for each combination of names (e.g. "John, Jane"). This creates much manual work to disentangle everything.

Describe the solution you'd like
It would be great to have a variant of the CATEGORY function which creates a copy for each value, instead of a scalar containing all values.

Start by tracing the export filename template handling for the CATEGORY function and how multiple tag values are currently combined into a directory name. Define the behavior for multiple values, then verify that exports place each image in the expected per-value directories without breaking the existing combination behavior.